The present invention relates to a solid-state imaging device, and more particularly to a solid-state imaging device, in which photoelectric conversion elements and other circuit elements are integrated on a semiconductor substrate.
A solid-state imaging device is required to have the same resolution as an image pickup tube used for the present television broadcasting. Hence, in the solid-state imaging device, a multiplity of photoelectric conversion elements (each for forming a pixel) are arranged so as to form a matrix having 500 rows and 500 to 1,000 columns, and scanning elements corresponding to the photoelectric conversion elements are disposed together with the photoelectric conversion elements. Accordingly, the solid-state imaging device is fabricated by using the MOSIC technology capable of integrating circuit elements at a high-density, and usually includes CCD's (namely, charge coupled devices) or MOS transistors in addition to the photoelectric conversion elements.
The above prior art will be expained below, with reference to the drawings.
FIG. 1 shows the circuit configuration of a conventional CCD-type solid-state imaging device. It is to be noted that only 2.times.2 pixels are shown in FIG. 1 for the sake of simplicity. In FIG. 1, reference numerals 611, 621, 631 and 641 designate photoelectric conversion elements (for example, photodiodes) for converting incident light into an electric charge, 66, 67 and 68 CCD's for transferring a signal charge, 901 and 902 driver transistors of source follower circuits, 903 and 904 load transistors of the source follower circuits, 501, 502 and 503 current buffer circuits, 504, 505 and 506 resistors, 507 and 508 capacitors, 509 and 510 switches, and 511 a power source. The circuit elements 501 to 511 make up a correlated double sampling circuit 500. Referring to FIG. 1, when light is incident on the photodiodes 611, 621, 631 and 641, a signal charge corresponding to an incident light quantity is stored in each photodiode. The electric charges stored in the photodiodes are successively transferred to the driver transistor 901 of source follower circuit through the CCD's 66, 67 and 68, and the output of the driver transistor 902 of another source follower circuit is applied to the correlated double sampling circuit 500. Then, the correlated double sampling circuit 500 delivers the difference between that output of the driver transistor 902 which is obtained before the signal charge contributes to the output of the transistor 902, and that output of the transistor 902 which is obtained after the signal charge has contributed to the output of the transistor 902. In more detail, the switches 509 and 510 are first turned on and turned off, respectively, to supply the capacitor 507 with that output of the transistor 902 which is obtained before the signal charge contributes to the output of the transistor 902. Next, the switches 509 and 510 are turned off and turned on, respectively, to lead that output of the transistor 902 which is obtained after the signal charge has contributed to the output of the transistor 902, to the capacitor 508. Thus, the capacitor 508 stores the difference between that output of the transistor 902 which is obtained before the signal charge contributes to the output of the transistor 902, and that output of the transistor 902 which is obtained after the signal charge has contributed to the output of the transistor 902. A sampling circuit of this kind is described in an article entitled "Characterization of Surface Channel CCD Image Array at Low Light Levels" by M. H. White et al. (IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. SC-9, No. 1, FEBRUARY, 1974, pages 1 through 13).
Next, a conventional MOS-type solid-state imaging device will be explained, with reference to FIG. 2. It is to be noted that only a single pixel is shown in FIG. 2 for the sake of simplicity. In FIG. 2, reference numeral 611 designates a photodiode equivalent to that shown in FIG. 1, 601 a transistor for amplifying a signal, 604 a switch, 605 a load resistor, 606 a power source, and 64 a signal line. Further, another switch 602 and another power source 603 make up a reset circuit. Referring to FIG. 2, when light is incident on the photodiode 611, a signal charge corresponding to an incident light quantity is generated in the photodiode 611. The signal charge thus obtained is converted by the amplifying transistor 601 into an electric current, which flows through the signal line 64 to reach an output terminal.
In the conventional imaging device of FIG. 1, the signal charge is transferred to an output amplifier through the CCD's 66, 67 and 68, without being converted into a current or voltage. Accordingly, a noise charge may be mixed with the signal charge when the signal charge is propagated through the CCD's 66, 67 and 68, and thus there arises a problem that a signal-to-noise ratio is decreased by the noise charge.
Specifically, in a case where part of incident light leaks through the crack of a shading film, there arises a serious problem that a noise charge due to the leakage light is introduced into the CCD's 66, 67 and 68 and thus a smear is generated.
Further, in the conventional imaging device of FIG. 2, a plurality of amplifying transistors 601 are disposed, and variations in output level of the transistor 601 (the so-called offset) due to variations in impurity concentration or surface states or energy states at the silicon-oxide interface existing under the gate of the transistor 601 are sent out, as they are. Thus, the above variations in output level are observed as a signal. Accordingly, noise which is called "fixed pattern noise" is generated.
In view of the above-mentioned problems of these prior arts, the present inventors have proposed a solid-state imaging device which can suppress the smear and is low in noise level, in a U.S. patent application Ser. No. 109,319 filed on Oct. 19, 1987.
An example of the above solid-state imaging device will be explained below, with reference to FIG. 3. FIG. 3 is a circuit diagram showing this example. Referring to FIG. 3, photoelectric conversion elements (for example, photodiodes) 1 store electric charges corresponding to incident light, and are arranged so as to form a two-dimensional matrix. A photodiode 1 is connected to a reset switch 3 and the gate of a pixel amplifier 4 through a vertical gate switch 2 which is controlled by a vertical gate line 5, and a horizontal gate switch 43 which is controlled by a horizontal gate line 51. The drain electrode of the pixel amplifier 4 is connected to a drain line 44, and the source electrode of the pixel amplifier 4 is connected to a load transistor 49 through a horizontal signal line 45, a read-out gate switch 47 and a vertical signal line 48. The circuit elements 4, 45, 47, 48 and 49 make up a source follower circuit. Now, explanation will be made of the operation of a single pixel which is selected by a horizontal scanning circuit (for example, horizontal register) 22 and a vertical scanning circuit (for example, vertical register) 21. A reset operation is first performed by the reset switch 3, and the source follower circuit including the pixel amplifier 4 is operated to deliver an output signal which is obtained in a reset period, from an output terminal 50. Next, the signal charge stored in the photodiode 1 is supplied to the pixel amplifier 4 through the vertical gate switch 2 and the horizontal gate switch 43, to deliver an output signal corresponding to the signal charge, from the output terminal 50. Thus, the read-out operation of one pixel is completed. As can be seen from the above explanation, the output signal obtained in the reset period and the output signal obtained at a time the signal charge is applied to the pixel amplifier, are time-sequentially delivered. By using the difference between these output signals, noise due to variations of offset of pixel amplifier and the 1/f noise of the pixel amplifier 4 can be readily suppressed.
In the imaging device of FIG. 3, however, each pixel includes four transistors. Hence, the area of the photodiode 1 included in each pixel is restricted, and thus it is impossible to improve the photo-sensitivity of the imaging device in a great degree.